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105hp out of a four-stroke 900cc parallel twin is really quite good.

Noteworthy, too, is the first production use of a camless valvetrain. This begs the question of why they chose a parallel twin - which does require inertial compensation - over a flat (boxer) twin which does not. Unusually, a camless valvetrain for a flat twin also gives you the freedom to route all the manifolds parallel to the plane of the pistons and crankshaft rather than perpendicular to it. This creates a truly flat package, giving vehicle designers far more leeway in where they put the engine. An extreme example would be an ICE-based version of GM's skateboard car.

Of course, two-cylinder engines do have some major disadvantages: the lower your cylinder count, the greater the interval between ignition events. This increases the torsional vibrations in the crankshaft, especially when the engine is also boosted. The combination of Sarazin centrifugal pendulum damper and a two-mass flywheel goes a long way toward isolating the rest of the driveline from these vibrations, but torque delivery is still not as smooth as in an engine with more cylinders. This tends to become noticeable as increased NVH (droning) at low RPM.

Regarding the turbocharger: in an inline four, whenever one cylinder is executing its exhaust stroke, another is executing its intake stroke. That means the mass flow of pressurized air through the intake manifold is nearly continuous. Things get even better for higher cylinder counts. For an inline three, the situation is a little worse but still quite acceptable because a well-designed intake manifold will produce favorable resonance.

In a parallel or flat four-stroke twin, however, the generously dimensioned intercooler has to buffer the fresh air mass for 180 degrees crankshaft until the same(!) cylinder is ready to take in fresh air during the subsequent downstroke. Air mass flows into the intercooler more or less continuously but out in discrete packets. Normally, this is considered suboptimal.

The whole thing works at all only because the turbo shaft has enough inertia to keep spinning in-between exhaust strokes without crossing its pump line. All this implies significant turbo lag at low RPM, so you'll want to keep the engine revving higher than a boosted inline three or four of the same total displacement. Revving high increases noise level and fuel consumption, offsetting some of the gains achieved by using fewer, larger cylinders.

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"105hp out of a four-stroke 900cc parallel twin is really quite good."

Depends what you're comparing to. I know of no other turbo-charged parallel twin for comparison. On the other hand for a naturally aspirated engine my benchmark for the parallel twin would be the recent release from Rotax Bombardier's Aprilia plant in Austria of the engine they manufacture for BMW's F800S motorcycle. This particular engine has a bore/stroke of 82mm x 75mm at a maximum piston speed of 20 m/s to generate 64.3kw or 86Hp against the superior 106hp turbo-charged 900cc described. Even so, I would have to know more before I start to be impressed. I should also perhaps say, in passing, that the index of horepower per litre is an artificial contrivance which originates probably from track racing when organisers were less sophisticated. Comparing engines by their capacity is somewhat deceiving also. But you see it done all the time. In fact the real metric that I have found that describes engine power is piston diameter or more accurately the total piston area of the piston(s). The other variable is the BMEP which can be vastly increased of course by forced induction as was done in this example of a turbo-charged engine.
But turbo-charging is not the be all of engine power. The current design of the F800S is not the end of the road for this naturally aspirated 800cc engine either. Had the Aprilia designers been tasked with providing 106Hp from their engine within the constraints of natural aspiration and 20m/s piston speeds they would likely take this approach. They would first have need to increase the bore diameter by the square root of 106/86 or 1.11 to give a new bore of 91mm. To maintain the swept capacity to 800cc the 75mm stroke would have to be reduced by the ratio 86/106 yielding a new stroke of 61mm. Finally the original 8000rpm would need to be upped by 106/86 in order to obtain the maximum 20 m/s piston speed as before, and that would turn out to be 9860rpm. Note that the 91/61 oversquare ratio is quite consistent with that currently used by Yamaha and Honda's single cylinder engines.
Nevertheless turbocharging is a good route also for thermal efficiency plus it can help produce power at much lower rpms for John Q Public. I am thinking that durability and survivability of the clutch disc will be much higher for withstanding fumbled shifts at 5500rpm than 9500rpm.

Finally the two cylinder Citroen 2CV (1948-1990) with its 602cc air cooled horizontally opposed engine still lives with enthusiastic support. The Geo Metro (Suzuki Swift) website - Threecylinders.com - has vehicle owners witnessing 48mpg with manual transmissions. The manufacturers have deserted this arena. Turbo's are perfect for passenger vehicles which need power mostly for acceleration purposes but I hope they do not pose reliability issues. Neither Nascar nor F1 are using turbo charged vehicles at the present time I have to note. I am sure this vehicle will be a great success.
T2

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@ T2 -

the BRP Rotax 804 is a great engine, but it was designed for motorcycle applications. That means it doesn't need to meet the emissions or noise regulations that a car engine does.

The only other turbocharged parallel twin I'm aware of is the fairly expensive Weber MPE 750 turbo. The on-road automotive version delivers 83hp @ 6000 RPM. Rinspeed used a Weber 750 for its see-through Exesis concept.

That works out to 111hp/L vs. 118hp/L for this Fiat SGE, which will presumably be sold in much greater numbers.

http://www.weber-motor.com/de/produkte/mpe-750/turbomotor/index.html

It's possible there are other examples of turbocharged parallel twins in the Japanese kei-jidosha market, but those would limited to 660cc. They probably wouldn't meet European or US automotive emissions regs without modifications.

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Hi rgseidl, Weber's is site interesting. I see they have a 750cc with 80kw which almost matches that from the 900cc on the Panda. I should also acknowledge your turbo info of which I was unaware. Until now my practical knowhow had been mostly limited to Hugh McInnes book on Turbochargers.

However for your statement
"This begs the question of why they chose a parallel twin - which does require inertial compensation - over a flat (boxer) twin which does not."
On the Heritage section of Honda Motorcycle website I gleaned the following --
With half the cylinder and head castings of the 500 Interceptor's V-4 engine, the (four) in-line CBR cost $1000 (in 2007 dollars) less to produce.--
which may answer your question.

You mentioned other artifacts of parallel twins. Torque resonance because the rocking couple set up by the 360 degree firing of the parallel twin I see as an acoustic engineering problem to be solved. I know the Prius has a damper component (two mass flywheel ?) on its crankshaft to mitigate the effects of transient torques felt in the cabin whenever the ICE is started up or shut down while the vehicle is in motion. A twin may need an inertial balancer as you suggested.

Then there is the marketing angle of whether the consumer will accept a two cylinder vehicle. As an entry level vehicle its anti-establishment stance of two cylinders in the face of the conformal four cylinder model may appeal to the younger generation. Certainly the turbo boosting will add a certain amount of cache to the younger age group of buyers.

I will say in conclusion that the real problem, the 800lb gorilla nobody wants to talk about, is the continued existence of stepped transmissions in the 21st century. Many problems of the ICE over an extended speed range go away when the series hybrid technique is employed. The architecture which does not include the 'boutique energy' source of course is the one I support. But for now, the use of a turbo to save the considerable mass and friction of two extra cylinders is a step in the right direction for less costly and complicated vehicles in the future.
T2

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@ T2 -

sure, flat engines are more expensive but they do offer superior NVH. Btw it turns out I had misunderstood what Fiat meant with their electro-hydraulic valvetrain. It's not camless after all, a picture really is worth 1000 words (HHNN ABG).

See my detailed interpretation of the inner workings plus a discussion on the NVH aspects and possible target markets in the comments section here:

http://www.greencarcongress.com/2007/09/fiat-panda-aria.html

As for the transmission, I'm not sure I agree. The issue isn't so much that there are discrete steps as the number of them and the gear ratios chosen. Especially with an automated dual-clutch transmission, you can get both excellent transmission efficiency when a gear is engaged and, long gear ratios plus early upshift points. Those force a boosted engine to operate more efficiently at higher load and lower RPM.

The trade-off is that you have less reserve torque on tap, so you have to kick down a gear or two to achieve good acceleration. The step change in engine revs introduces a noticeable torsional jolt into the driveline. You'll want to keep the reduced polar moment of the cranktrain as small as possible and/or use an electric motor to adjust engine speed during rather than after the gear change.

The alternative is a CVT, either a belt type or a cone ring design:

http://www.ecurie-aix.rwth-aachen.de/downloads/krganimation.mpg

A full series hybrid driveline is heavy, expensive and not very efficient. It doesn't need a full ICE designed for rapid load response. A hermetically sealed free piston Stirling engine with linear alternators could be a better option.

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@T2 -

btw, the automotive version of the Weber 750 turbo delivers 61kW (= 83hp, = 111hp/L). The leisure version does not meet automotive emissions. Perhaps you misread the data sheet?

http://www.weber-motor.com/de/produkte/mpe-750/turbomotor/index.html

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@Rafael
Yes, my eye must have caught the specs for the off highway version and I honed in on that instead.

By the way I did some math on the BMW F800S. With its 75mm stroke and top speed of 8000rpm that works out to an extreme piston speed of 20m/s so they are dancing with the devil on that one !
I've noticed that Japanese engines almost never touch that boundary even the 13,500rpm CBR600.
T2

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I would love to introduce my patented multi-valve pneumatically activated engine, which runs in two-stroke mode or four-stroke mode, depending on the required power output, with programmable variable valve timing.
Please provide the appropriate contacts.

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